Spectrophotometric analysis is based on the absorption or attenuation of electromagnetic radiation of a particular wavelength by the sample under test. The region of the electromagnetic spectrum most useful for spectrophotometric analysis is that between 2,000 A and 300 microns. The instruments used for conducting such analysis are referred to as spectrophotometers. A simple spectrophotometer consists of a source of radiation; a monochromator containing a prism, grating, narrow band filter, or the like, which disperses the radiation from the source so that only a limited wavelength range is allowed to pass through the sample; a cell containing the sample; and a detector, such as a photocell, which measures the amount of light transmitted through the sample.
Any light reaching the detector which does not pass through the sample itself is considered to be "stray" light, and this stray light produces errors in the readings by the detector. Stray light can result from light which enters the side wall of the sample cell at the source end and which leave at the detector end, with the side wall acting as a light pipe. Stray light can also result when the light entering the sample cell is not completely collimated or is scattered by the sample, so that some of the light enters the side wall of the cell at the proper angle to establish a semi-light pipe condition in the side wall.
Stray light can become a significant problem in many spectrophotometric analyses. For example, in some spectrophotometric applications, high light attenuation by the sample of the order of 1:10.sup.3 - 1:10.sup.4 are encountered. In such applications, even if only one part in 10.sup.5 of the main light beam enters the side wall of the cell to be conducted thereby to the detector, errors of the order of 1-10% may result in the detector reading.